The present invention relates generally to the field of solid state electrochemical devices, and more particularly to cell to cell interconnects used therein. Interconnects of the prior art are typically made of metal or metal alloys or of an oxide such as doped lanthanum chromite and are typically about 0.2 mm thick. Interconnects may also serve as current collectors for solid state electrochemical devices.
Current collectors for solid state electrochemical devices have been described in the prior art, for example in Hatchwell et al., J. Power Sources, 70 (1998) 85-90, the contents of which are hereby incorporated by reference in their entirety.
Solid state electrochemical devices are often used as cells and include two porous electrodes, the anode and the cathode, and a dense solid electrolyte and/or membrane which separates the electrodes. For the purposes of this application, unless otherwise explicit or clear from the context in which it is used, the term xe2x80x9celectrolytexe2x80x9d should be understood to include solid oxide membranes used in electrochemical devices, whether or not potential is applied or developed across them during operation of the device. In many implementations, such as in fuel cells and oxygen and syn gas generators, the solid membrane is an electrolyte composed of a material capable of conducting ionic species, such as oxygen ions, or hydrogen ions, yet has a low electronic conductivity. In other implementations, such as gas separation devices, the solid membrane is composed of a mixed ionic electronic conducting material (xe2x80x9cMIECxe2x80x9d). In each case, the electrolyte/membrane must be dense and pinhole free (xe2x80x9cgas-tightxe2x80x9d) to prevent mixing of the electrochemical reactants. In all of these devices a lower total internal resistance of the cell improves performance.
The ceramic materials used in conventional solid state electrochemical device implementations can be expensive to manufacture, difficult to maintain (due to their brittleness) and have inherently high electrical resistance. The resistance may be reduced by operating the devices at high temperatures, typically in excess of 900xc2x0 C. However, such high temperature operation has significant drawbacks with regard to the device maintenance and the materials available for incorporation into a device, particularly in the oxidizing environment of an oxygen electrode, for example.
The preparation of solid state electrochemical cells is well known. For example, a typical solid oxide fuel cell (SOFC) is composed of a dense electrolyte membrane of a ceramic oxygen ion conductor, a porous anode layer of a ceramic, a metal or, most commonly, a ceramic-metal composite (xe2x80x9ccermetxe2x80x9d), in contact with the electrolyte membrane on the fuel side of the cell, and a porous cathode layer of a mixed ionically/electronically-conductive (MIEC) metal oxide on the oxidant side of the cell. Electricity is generated through the electrochemical reaction between a fuel (typically hydrogen produced from reformed methane) and an oxidant (typically air). This net electrochemical reaction involves charge transfer steps that occur at the interface between the ionically-conductive electrolyte membrane, the electronically-conductive electrode and the vapor phase (fuel or oxygen). The contributions of charge transfer step, mass transfer (gas diffusion in porous electrode), and ohmic losses due to electronic and ionic current flow to the total internal resistance of a solid oxide fuel cell device can be significant. Moreover, in typical device designs, a plurality of cells are stacked together and connected by one or more interconnects. Resistive loss attributable to these interconnects can also be significant. It is the subject of this invention to provide new and improved interconnects.
In work reported by de Souza, Visco, and De Jonghe (de Souza, S.; Visco, S. J.; De Jonghe, L. C. Reduced-temperature solid oxide fuel cell based on YSZ thin-film electrolyte. Journal of the Electrochemical Society, vol.144, (no.3), Electrochem. Soc, March 1997. p.L35-7. 7), the contents of which are incorporated by reference in their entirety for all purposes, a thin film of yttria stabilized zirconia (YSZ) is deposited onto a porous cermet electrode substrate and the green assembly is co-fired to yield a dense YSZ film on a porous cermet electrode. A thin cathode is then deposited onto the bilayer, fired, and the assembly is tested as an SOFC with good results. In work reported by Minh (Minh, N. Q. (Edited by: Dokiya, M.; Yamamoto, O.; Tagawa, H.; Singhal, S. C.) Development of thin-film solid oxide fuel cells for power generation applications. Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells (SOFC-IV), (Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells (SOFC-IV), Proceedings of Fourth International Symposium Solid Oxide Fuel Cells, Yokohama, Japan, 18-23 Jun. 1995.) Pennington, N.J., USA: Electrochem. Soc, 1995. p. 138-45), a similar thin-film SOFC is fabricated by tape calendaring techniques to yield a good performing device.
Solid state electrochemical devices are becoming increasingly important for a variety of applications including energy generation, oxygen separation, hydrogen separation, coal gasification, and selective oxidation of hydrocarbons. These devices are typically based on electrochemical cells with ceramic electrodes and electrolytes and have two basic designs: tubular and planar. Tubular designs have traditionally been more easily implemented than planar designs, and thus have been preferred for commercial applications. However, tubular designs provide less power density than planar designs due to their inherently relatively long current path that results in substantial resistive power loss. Planar designs are theoretically more efficient than tubular designs, but are generally recognized as having significant safety and reliability issues due to the complexity of sealing and manifolding a planar stack.
The current collectors of the prior art, see U.S. Pat. No. 6,214,061, the contents of which are hereby incorporated by reference in their entirety, form a physically rigid layer of material. U.S. Pat. No. 3,377,203 discloses a method for producing fuel cells of solid electrolyte and ceramic oxide electrode layers by sintering the electrode layers to the electrolyte. U.S. Pat. No. 4,767,518 discloses a solid oxide electrode (anode) made of metal particles that are immobilized by stabilized zirconia which may also contain praseodymium (Pr). The Pr may be added in the form of a solution. U.S. Pat. No. 4,885,078 discloses an electrochemical device which may be a solid oxide cell which comprises a porous electrode containing a deposit of metal oxide or metal salt capable of forming metal oxide upon heating, where the metal may be Pr. U.S. Pat. No. 5,021,304 discloses a method of coating a separate electronically conducted layer on a porous electrode having the steps of applying a mixture of metal salts including nitrates to the electrodes with a surfactant, and heating to form the oxides. Pr oxide is included in a list of dopant oxides which may be used. U.S. Pat. No. 6,416,897 discloses tubular screen electrical connection supports for solid oxide fuel cells comprising porous, hollow, electrically conducting metal mesh conductors between fuel cells that are made of nickel. U.S. Pat. No. 5,258,240 discloses electrical connectors for fuel cells comprising felts or foils.
The total amount of energy produced by a solid state fuel cell can be increased by stacking the cells anode to cathode. Interconnect layers are placed between the anode and cathode layers of adjacent cells to provide electrical connection from cell to cell and to serve as a barrier between the cells to prevent the migration of fuel or oxidant therebetween.
This invention contemplates a novel cell to cell interconnect and current collector and process of making such devices useful for cell to cell interconnection at high temperatures, i.e. greater than 600 degrees centigrade. This invention contemplates devices comprising stacks of plates or bundles of tubes which are electrically connected. Typical prior art devices are connected in series and/or parallel by expensive metal oxides or by expensive metals such as Pt or Ni (reducing atmosphere). The interconnect and current collectors of this invention are comprised of a metal screen or metal felt coated with a perovskite.
In other aspects, the invention provides devices in accordance with the present invention tailored to specific purposes, for example, oxygen generators, gas separators, solid oxide fuel cells and syn gas generators.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.